FT Viggen - expansion chamber

[ladykate]

Experimenting with 12 blade fans with the Viggen. They work fine but are soooooo quiet that it isn't worth the effort. The sound is right - just isn't loud. Has anyone experimented with the expansion chamber to get different sounds (especially with a 12 blade)?

Specifically, I want to know if increasing the expansion chamber size at the rear will get me the whoosh that I'm looking for. I know about the change in thrust vs speed. Looking for sound.

 

[El gato]

I just finished a Viggen, but have not had a chance to fly it yet due to weather. Anyhow to change the sound you may want to add some turbulence to the exhaust. If you add some outlet guide vanes to the exhaust you may be able to get more of the sound you want, perhaps at the expense of a small amount of thrust.

Just a thought.

 

[ladykate]

I just finished a Viggen, but have not had a chance to fly it yet due to weather. Anyhow to change the sound you may want to add some turbulence to the exhaust.

Just a thought.

I noticed you put that in a 5 blade. You probably don't need it in a 5 blade since that one has a great sound... loud and nice.

Also, even the slight change in airflow that a thrust vectoring setup does would indicate that your airframe would screw through the sky with this setup. I would NOT do it that way. However, you may have a point - perhaps turbulence vanes that are offsetting (like on the wings of a 3D airplane) might work.

The 12 blade is extremely quiet. I might try it on my 12 blade to see if it changes anything. Has anyone else tried putting turbulence vanes in the expansion chamber?

 

[El gato]

The vanes I put in were to demonstrate the idea. They are not attached to anything, I held them there with my fingers and listened to the sound as I throttled the motor. And yes it is loud with the 5 blade.

Well, now you have me interested in trying the vanes just to see what happens. I noticed no tendency to roll the A/C when throttled to full power, but it may be different when I fly it.

If you could access the exhaust tube how about perforations in the tube and then put another tube around it but the two would be separated ( two concentric tubes) allowing the airflow to escape.

Or you could try a convergent/divergent duct.

An interesting experiment.

 

[ladykate]

I glued some Popsicle sticks in the back of my 12 blade. Running it up in the shop. It seemed a little louder but that could just be me. I'll take it out today and try it.

BTW: I reversed the airflow on the sticks so that it was balanced. Again, like the turbulators on a wing.

 

[El gato]

Good luck on the experiment, it will be interesting to see how it works.
Another idea came to me, a rod thru the exhaust tube with a piece of vinyl tape on it to flap like a flag in the wind ( flutter )

Let us know how you make out.

 

[ladykate]

Good luck on the experiment, it will be interesting to see how it works.
Another idea came to me, a rod thru the exhaust tube with a piece of vinyl tape on it to flap like a flag in the wind ( flutter )

Let us know how you make out.

It didn't work very well. There was a modicum of whoosh but not nearly what I was looking for. There has to be a way. I think the expansion chamber is the key.... just how to do it is the question.